The present invention relates to a method of fabricating a high-density magnetooptic recording medium.
2. Description of the Prior Art
A magnetooptic recording/reproducing system forms information recording bits, namely, magnetic bubble domains by locally heating a magnetooptic recording medium with a laser beam, and reads out the recorded information by utilizing magnetooptic effect, namely, Kerr effect or Faraday effect. Accordingly, the recording bits must be miniaturized to increase recording density in magnetooptic recording. However, problems arises in reproducing resolution when recording density is increased. Resolution is dependent on the wavelength of the laser beam used for reproducing, and the numerical aperture N.A. of the objective lens.
A general magnetooptic recording/reproducing system will be described with reference to FIGS. 3A to 3D. A method of reproducing binary information recorded in a magnetooptic recording medium 3, such as a magnetooptic disk, will be described. In FIG. 3A, indicated by shaded circles are recording bits 4 formed in a land formed between grooves 1. Suppose that a laser beam is focused in a circular spot 5 on the magnetooptic recording medium. Then, if the recording bits 4 are formed so that only one recording bit 4 is included in the spot 5 as shown in FIG. 3A, the presence or absence of a recording bit 4 in the spot 5 designates either of two state, namely, a digital 1 or a digital 0, as shown in FIG. 3B or 3C. Accordingly, if the recording bits 4 are formed at equal intervals, the magnetooptic recording/reproducing system provides a reproducing output signal having, for example a sinusoidal waveform as shown in FIG. 3D.
If recording bits 4 are formed at a very high bit density as shown in a typical top plan view of a recording pattern in FIG. 4A, it is possible that the spot includes a plurality of recording bits 4. Suppose that two adjacent recording bits among three successive recording bits 4a, 4b and 4c are included in the spot 5. A state in which the two recording bits 4a and 4b are included in the spot 5 as shown in FIG. 4B and a state in which the two recording bits 4b and 4c are included in the spot 5 as shown in FIG. 4C cannot be discriminated from each other, because a reproduced output signal in the state shown in FIG. 4B and a reproduced output signal in the state shown in FIG. 4C are equal to each other. Therefore, the reproduced output signals form, for example, a straight line as shown in FIG. 4D.
Since the conventional magnetooptic recording/reproducing system thus reads the recording bits 4 as recorded on the magnetooptic recording medium 3, restrictions on the reproducing resolution cause problems in S/N (C/N) even if recording bits can be formed in a high bit density, and hence the conventional magnetooptic recording/reproducing system is unable to achieve high-density recording and reproducing.
The reproducing resolution dependent on the wavelength of the laser beam and the numerical aperture of the lens must be improved to solve the problems in S/N (C/N). The applicant of the present patent application proposed a magnetooptic recording/reproducing system capable of reproducing recorded information in a very high resolution in, for example, Japanese Patent Application No. Hei 1-225685 titled "Magnetooptic Recording/Reproducing Method".
This previously proposed magnetooptic recording/reproducing system reads only the recording bit 4 of a temperature in a predetermined temperature range on a magnetooptic recording medium by utilizing a temperature distribution formed on a magnetooptic recording medium formed by the movement of a reproducing spot 5 relative to the magnetooptic recording medium in reproducing recorded information to enhance the resolution. Magnetooptic recording systems may be classified into those of a so-called emergence type and those of an extinction type.
The magnetooptic recording system of an emergence type will be described with reference to FIGS. 5A and 5B. FIG. 5A is a typical top plan view of a recording pattern formed on a magnetooptic recording medium 10, and FIG. 5B is a typical sectional view showing a state of magnetization of the magnetooptic recording medium 10. As shown in FIG. 5A, the magnetooptic recording medium 10 moves in the direction of an arrow d relative to a spot 5 of a laser beam. As shown in FIG. 5B, the magnetooptic recording medium 10 is, for example, a magnetooptic disk having at least a reproducing layer 11 and a recording layer 13 formed of perpendicularly magnetizable films, respectively, desirably, the reproducing layer 11, the recording layer 13 and an intermediate layer 12 formed between the reproducing layer 11 and the recording layer 13. Arrows in the layers 11, 12 and 13 in FIG. 5B indicate the direction of magnetic moment typically. In FIG. 5B, magnetic domains indicated by the downward arrows are in an initial state, for example, a 0 state or a 1 state, and those indicated by the upward arrows are recording bits 4 in a 1 state or 0 state formed at least in the recording layer 13.
In reproducing recorded information signals from the magnetooptic recording medium 10, an external initializing magnetic field H.sub.i is applied to the magnetooptic recording medium 10 to magnetize the reproducing layer 11 downward, as viewed in FIG. 5B, for initialization. Although the recording bits 4 of the reproducing layer 11 are extinguished by initialization, the respective directions of magnetization of regions in the reproducing layer 11 and the recording layer 13 corresponding to the recording bits 4 are maintained reverse to each other by magnetic domain walls formed in the intermediate layer 12, so that the recording bits 4 remain in latent recording bits 41.
A reproducing magnetic field H.sub.r of a direction reverse to that of the initializing magnetic field H.sub.i is applied at least to the reproducing regions of the magnetic recording medium 10. As the magnetic recording medium 10 moves, the region having the initialized latent recording bit 41 comes under the spot 5. Then, a high-temperature region 14 is formed in the front side of the spot 5 as indicated by a shaded area enclosed by a broken line a. In the high-temperature region 14, magnetic domain walls in the intermediate layer 12 disappear, the magnetization of the recording layer 13 is copied into the reproducing layer 11 by exchange force, so that the latent recording bit 41 in the recording layer 13 emerges in the reproducing layer 11 in a reproducible recording bit 4.
Accordingly, the recording bit 4 can be read by detecting the rotation of the plane of polarization of the spot 5 caused by Kerr effect or Faraday effect corresponding to the direction of magnetization of the reproducing layer 11. Latent recording bits 41 in a low-temperature region 15, other than the high-temperature region 14, in the spot 5 do not emerge into the reproducing layer 11, and hence the reproducible recording bit 4 is included only in the narrow high-temperature region 14. therefore, even if information is recorded in a high recording density on the magnetooptic recording medium 10 capable of high-density recording, in which a plurality of recording bits 4 are included in the spot 5, only one of the recording bits 4 can be read for high-resolution signal reproducing.
To carry out signal reproducing in such a mode, the initializing magnetic field H.sub.i, the reproducing magnetic field H.sub.r, the respective coercive forces, values of thickness, intensities of magnetization and values of domain wall energy of the magnetic layers are determined selectively according to the temperature of the high-temperature region 14 and that of the low-temperature region 15. The coercive force H.sub.c1, thickness h.sub.1 and saturation magnetization M.sub.s1 of the reproducing layer 11 must meet an expression (1) to initialize only the reproducing layer 11. EQU H.sub.i &gt;H.sub.c1 +.sigma..sub.w2 /2M.sub.s1 h.sub.1 ( 1)
where .sigma..sub.w2 is the interfacial domain wall energy between the reproducing layer 11 and the recording layer 13.
An expression (2) must be met to maintain the information recorded in the recording layer 13 by the magnetic field. EQU H.sub.i &lt;H.sub.c3 -.sigma..sub.w2 /2M.sub.s3 h.sub.3 ( 2)
where H.sub.c3 is the coercive force, M.sub.s3 is the saturation magnetization and h.sub.3 is the thickness of the recording layer 13.
An expression (3) must be met to maintain the magnetic domain walls formed in the intermediate layer 12 between the reproducing layer 11 and the recording layer 13 after the initializing magnetic field H.sub.i has been applied to the magnetooptic recording medium 10. EQU H.sub.c1 &gt;.sigma..sub.w2 /2M.sub.s1 h.sub.1 ( 3)
An expression (4) must be met to heat the high-temperature region 14 at a selected temperature T.sub.H. EQU H.sub.c1 -.sigma..sub.w2 /2M.sub.s1 h.sub.1 &lt;H.sub.r &lt;H.sub.c1 +.sigma..sub.w2 /2M.sub.s1 h.sub.1 ( 4)
The magnetization of the latent recording bits 41 of the recording layer 13 can be copied into the reproducing layer 11 only in regions corresponding to the magnetic domain walls of the intermediate layer 12 to form recording bits 4 in the reproducing layer by applying the reproducing magnetic field H.sub.r meeting the expression (4).
Although the magnetooptic recording medium 10 employed by this magnetooptic recording/reproducing system has the reproducing layer 11, the intermediate layer 12 and the recording layer 13, the magnetooptic recording/reproducing system may employ a four-layer magnetooptic recording medium additionally provided with an auxiliary reproducing layer 17 between the reproducing layer 11 and the intermediate layer 12 as shown in an enlarged typical sectional view in FIG. 6. The auxiliary reproducing layer 17 supplements the characteristics of the reproducing layer 11 to compensate the coercive force of the reproducing layer 11 at a room temperature to stabilize the magnetization of the reproducing layer 11 caused by the initializing magnetic field H.sub.i regardless of the existence of magnetic domain walls and to decrease the coercive force sharply at a temperature near the reproducing temperature so that the magnetic domain walls of the intermediate layer 12 expand into the auxiliary reproducing layer 17 to finally invert the reproducing layer 11 and to extinguish the magnetic domain walls for satisfactory emergence of the recording bits 4.
The coercive force H.sub.cl of the reproducing layer 11 of a four-layer magnetooptic recording medium provided with the auxiliary reproducing layer 17 is substituted b H.sub.CA expressed by an expression (5) and the term .sigma..sub.w2 /M.sub.s1 h.sub.1 is substituted by .sigma..sub.w2 /(M.sub.s1 h.sub.1 +M.sub.ss h.sub.s) EQU H.sub.CA =(M.sub.s1 h.sub.1 H.sub.c1 +M.sub.ss h.sub.s H.sub.cs)/(M.sub.s1 h.sub.1 +M.sub.ss h.sub.s) (5)
where H.sub.c1 &lt;H.sub.CA &lt;H.sub.cs for the magnetooptic recording/reproducing system of an emergence type, and M.sub.ss, h.sub.s and H.sub.cs are the saturation magnetization, coercive force and thickness, respectively, of the auxiliary reproducing layer 17.
The magnetooptic recording/reproducing system of an extinction type will be described hereinafter with reference to FIGS. 7A and 7B. FIG. 7A is a typical top plan view of a recording pattern formed on a magnetooptic recording medium 10, and FIG. 7B is a typical sectional view showing a state of magnetization, in which parts like or corresponding to those shown in FIGS. 5A and 5B are denoted by the same reference characters and the description thereof will be omitted.
This magnetooptic recording medium 10 does not need the initializing magnetic filed H.sub.i. In reproducing information recorded on the magnetooptic recording medium 10, the high-temperature region 14 is heated so that an expression (6) is satisfied, and then an external reproducing magnetic field H.sub.r is applied to the magnetooptic recording medium 10 to extinguish recording bits 4 in the high-temperature region 14 included in the spot 5 of a laser beam in the reproducing layer 11 magnetized downward as viewed in FIG. B. EQU H.sub.r &gt;H.sub.c1 +.sigma..sub.w2 /2M.sub.s1 h.sub.1 ( 6)
Thus, the magnetooptic recording/reproducing system of an extinction type enables information stored only in the recording bits 4 in the low-temperature region 15 in the spot 5 to be reproduced to improve the resolution. The conditions including the coercive force are determined so that the recording bits 4 of the recording layer 13 remain in latent recording bits 41 in an extinction state and the magnetization of the recording layer 13, i.e., the recording bits 4, are copied into the reproducing layer 11 and held therein in a reproducible state at a room temperature.
These magnetooptic recording/reproducing systems of an emergence type and an extinction type reproduce the recording bit in a local region included in the spot of the reproducing laser beam to reproduce the information in an enhanced resolution.
It is also possible to reproduce recorded information by a magnetooptic recording/reproducing system of a combined type having functions of both the magnetooptic recording/reproducing system of an emergence type and the magnetooptic recording/reproducing system of an extinction type. The magnetooptic recording/reproducing system of a combined type forms a high-temperature region 14, a middle-temperature region 16 and a low-temperature region 15 in the front portion, middle portion and rear portion with respect to the direction of movement of the magnetooptic recording medium relative to a spot 5, respectively, of the spot 5 as shown in FIG. 8, and utilizes the high-temperature region 14 for the functions of the extinction type described with reference to FIGS. 7A and 7B, and the middle-temperature region 16 and the low-temperature region 15 for the functions of the emergence type described with reference to FIGS. 5A and 5B.
This magnetooptic recording/reproducing system of a combined type is able to make only a recording bit 4, a shaded circle in FIG. 8, included in the narrow middle-temperature region 16 between the high-temperature region 14 and the low-temperature region 15 emerge into the reproducing layer 11. Accordingly, the magnetooptic recording/reproducing system of a combined type is capable of reproducing recorded information in a high resolution.
Thus, the magnetooptic recording/reproducing systems are able to reproduce recorded information in a very high resolution regardless of the wavelength .lambda. of the laser beam and the numerical aperture N.A. of the objective lens. Accordingly, the magnetooptic recording system need not use a reading light beam of a particularly short wavelength and is able to determine the wavelength of a reading light beam taking into consideration magnetooptic effect, heating effect, and the sensitivity of the optical detector. That is, the magnetooptic recording/reproducing system is able to reproduce recorded information in a high resolution even if the same uses a semiconductor laser beam having a comparatively large wavelength, such as 780 nm.
The magnetooptic recording/reproducing system capable of reproducing recorded information in a very high resolution enables high-density recording, namely, miniaturization of recording bits and reduction of the pitch of recording bits.
If minute bits are formed by using such a reproducing semiconductor laser beam for recording, namely, if minute bits are formed by the same spot as that for reproducing (reading), the magnetooptic recording medium must be heated by the laser beam in a temperature distribution having a peak corresponding to a temperature T.sub.w capable of forming a recording bit, for example, the Curie temperature, and the recording bit must be formed in a width corresponding to a small region .phi..sub.p as shown in FIG. 9, which requires strict conditions for materials forming the magnetic layers of the magnetooptic recording medium and the power of the recording laser beam. Such strict conditions make the enhancement of recording density difficult as compared with the improvement of the resolution of the magnetooptic recording/reproducing system.